Dynamical friction of merged satellites: quantum Mach number as a function of the relative orbital distance for all merged satellites in the entire TNG50 sample between z = 2 and z = 0 for m₂₂ = 0.3. The hexagonal bins represent regions containing at least one satellite. The red dashed line shows the adopted boundary between the FDM and CDM regimes, based on the quantum Mach number defined in Equation (9). The orange dashed line indicates the threshold where DM dynamical friction becomes efficient, as defined in Equation (14). The magenta box highlights the region of interest where FDM dynamical friction must be taken into account, but this only concerns 1.8% of the satellites.